The monitoring and/or determination of the concentration of artificial aerosol alpha activity in the air is absolutely necessary in plants which are engaged in the manufacture and preparation of fuel elements The measurement of these values could readily be carried out if natural aerosol activity concentrations such as decay products of radon and thoron were not present in the air. Radon and thoron decay products deposit on aerosols and are then measured as alpha activity. Complicating this, the natural radioactivity concentration can vary by a factor of 10 during the course of the day depending on meteorological conditions.
Because of these circumstances it is generally known, in order to carry out monitoring and/or determination of artificial radioactivity levels, to attempt to suppress the natural aerosol alpha activity concentration as far as possible, without thereby suppressing the artificial aerosol alpha activity concentration.
Two different methods and apparatus are known for determining the artificial aerosol alpha activity concentrations in the air, namely the ABPD (Alpha Beta Pseudocoincidence Difference) method and the APIA (Alpha Particles In Air) monitoring method. The latter is an alpha spectroscopic measurement method.
In the ABPD method, the deposits on aerosols, acting as alpha radiators, in the case of the radon series, RaA, RaC' and RaC, as well as the thoron decay products, ThA, ThC' and ThC, are used in order, by specific measurement of the RaC and RaC' as well as the ThC and ThC', to compensate for the natural alpha activity. For measuring RaC and RaC' specifically, the fact is utilized that upon a beta decay of RaC within the half-life of 160 milliseconds, an alpha decay of RaC' will take place with a probability of 50%. Since ThC' has a half-life of only 0.3 milliseconds, an alpha decay of ThC' takes place within 160 milliseconds practically of every beta decay. These successive beta-alpha decays are referred to as pseudocoincidences and represent a characteristic property of radon and thoron decay products. By the measurement of the pseudocoincidences the natural alpha activity can be determined, and by subtraction of these values from the values of the total alpha activity, the artificial alpha aerosol activity concentration can be determined.
Despite the advantages of the ABPD method, namely continuous online measurement, simultaneous measurement of aerosol alpha and beta activity concentrations with a low limit of detection, and large air flow (40 to 50 m.sup.3 /per hour); there are also considerable disadvantages. The parameters of the external local conditions, such as weather, temperature, age of the air masses, local geology, etc., enter into the factors that must be taken into consideration, in order to compensate for the natural activity concentrations and thereby determine the result of the measurement of the artificial activity concentrations. The factors must be determined "on the spot". Similarly, extreme conditions such as rapid changes in the ventilation and changes in weather conditions must be measured by tests extending over several weeks. If the parameters of the local conditions change, then the ABPD method cannot, of course, respond rapidly, and there is therefore a slow adaptation to the changes.
The APIA method is an alphaspectroscopic method with high energy discrimination, by which many difficulties in the compensation of natural and artificial aerosol activity concentrations have been eliminated. The APIA method also has disadvantages, however, such as the low air flow of about 2,000 liters per hour, and the complicated and thus expensive construction of the dusting device and of the measurement chamber, since the detector must be operated under vacuum. Furthermore, the detector is exposed, unprotected, to the medium to be measured within the measurement space. Since this medium may also contain corrosive gases, it is possible for the use or the working life of the detector to be drastically limited.